The present invention generally relates to thermometric cells adapted to calibrate thermometers and, more specially, a sealed thermometric cell including multichambers which connect various fixed points.
The development, in 1975, by a team to which the present inventors belong, of the first sealed cell using the triple point of argon as a transfer reference has allowed to greatly improve, both as far as the accuracy and process easiness are concerned, cryogenic calibrations.
Calibrating within wider ranges, however, requires using further references, typically characteristic phase transitions, with a view to limiting interpolations or extrapolations when calibrating thermometers. Now, as it is known, filling a cell with a high purity substance having such characteristic phase transition is an extremely delicate and meticulous work. Furthermore, using several sealed cells, each including one of the standard substances thereof, can hardly be contemplated because of implementing difficulties and as controlling cryostats is critical and the volume they provide has a low value.
Therefore, there is a need for a thermometric cell capable of connecting different fixed points, and thus allowing to calibrate a thermometer for said different fixed points without having to dismantle either the equipement nor the thermometer when carrying out successive calibrations.
Now, to this end, several parameters should be taken into account. Geometry of individual cells should be determined, not only in order to obtain a miniaturized cell, but also to optimize the individual cell envelope capacity and the response time thereof. Also, due to the extreme accuracy required by such calibrations, cell cleaning problems (in order to avoid, specially, contaminating the pure calibration substance with the cell material) should lead to a trade-off between good heat exchange characteristics and said cleaning capability (one should remember here that a 10 ppm impurity rate is already on the high side).